1. Field of the Invention
The present invention relates to a nonreciprocal circuit device for use in a mobile communications apparatus such as a portable telephone, etc., and particularly to a nonreciprocal circuit device used as a circulator or an isolator in a high frequency band such as the microwave band or the like.
2. Description of the Related Art
Recently in mobile communications, high frequency apparatus has increasingly been miniaturized and generalized, and it has also been strongly demanded to decrease the size and cost of a nonreciprocal circuit device used in such apparatus.
A known nonreciprocal circuit device is, for example, a device comprising a plurality of central electrodes arranged to cross each other in an electrically isolated state, microwave magnetic materials provided above and below the plurality of central electrodes, and a permanent magnet for applying a DC magnetic field to the plurality of central electrodes, i.e., a lumped-parameter nonreciprocal circuit device. Such a lumped-parameter nonreciprocal circuit device is used as, for example, a circulator or an isolator.
FIG. 1 is an exploded perspective view showing an example of a conventional circulator. In this circulator, in order to decrease the size thereof, a plurality of central conductors are arranged in a ferromagnetic body 1 to cross each other in an electrically isolated state. Namely, as shown in an exploded perspective view of FIG. 2, the ferromagnetic body 1 has a laminated structure comprising a plurality of ferromagnetic material layers 1a to 1e. On the upper sides of the ferromagnetic material layers 1b, 1c and 1d are formed central conductors 2a and 2b, 2c and 2d and 2e and 2f, respectively. In other words, on the upper side of each of the ferromagnetic material layers 1b to 1d are arranged a pair of central conductors.
The central conductors 2a and 2b, the central conductors 2c and 2d, and the central conductors 2e and 2f are arranged to cross each other in a laminated state, and are electrically isolated by the ferromagnetic material layers 1c and 1d.
On the ferromagnetic material layers 1a and 1e are formed earth electrodes 3a and 3b, respectively.
In the ferromagnetic body 1 shown in FIG. 1, external electrodes 4a, 4b and 4c are formed on the side thereof so as to be connected to the earth electrodes 3a and 3b in common, and each external electrode being electrically connected to the ends of one of the pairs of central conductors 2a-2f.
In FIG. 1, to the upper side of the ferromagnetic body 1 is fixed a dielectric body 5. The dielectric body 5 comprises dielectric ceramic and has a capacitor provided therein for forming a matching circuit. Namely, as shown in an exploded view of FIG. 3, the dielectric body 5 has a laminated structure comprising dielectric layers 5a and 5b. On the upper side of the dielectric layer 5a are formed capacity electrodes 6a, 6b and 6c. On the upper side of the dielectric layer 5b is formed an earth electrode 7. Therefore, in each portion where the capacity electrodes 6a to 6c are overlapped with the earth electrode 7 and the earth electrode 3b through the dielectric layers 5b and 5a, respectively, a capacitor is formed.
In FIG. 1, external electrodes 8a, 8b and 8c are formed on the side of the dielectric body 5. Each of these external electrodes 8a to 8c is electrically connected to one of the capacity electrodes or one of the earth electrodes.
On the other hand, the ferromagnetic body 1 and the dielectric body 5 are contained in a terminal plate 9 having a cylindrical concave 9a at the center thereof. In the terminal plate 9 are formed conductor patterns 10a to 10c which constitute input/output terminals, and conductive patterns 10d, 10e and 10f which are connected to the earth potential.
The external electrodes 4a to 4c formed on the side of the ferromagnetic body 1 and the external electrodes 8a to 8c formed on the side of the dielectric body 5 are contained in the concave 9a of the terminal plate 9 and appropriately connected to the conductive patterns 10a to 10f.
In FIG. 1, a permanent magnet 11 is provided for applying a magnetic field to a portion in the ferromagnetic body 1 where the central conductors cross each other. The nonreciprocal circuit device shown in FIG. 1 further comprises metallic yokes 12 and 13. The terminal plate 9 and the magnet 11 are held between the yokes 12 and 13. The yokes 12 and 13 constitute a magnetic circuit for applying a magnetic field together with the magnet 11.
In the nonreciprocal circuit device shown in FIGS. 1 to 3, since the portion where the plurality of central conductors 2a, 2b to 2e and 2f cross each other in an electrically isolated state is integrally formed by using the ferromagnetic body 1, the nonreciprocal circuit device can easily be manufactured, and miniaturized.
However, since the ferromagnetic body 1 and the dielectric body 5 are separately fired and then joined together, the external electrodes 4a to 4c and the external electrodes 8a to 8c on the sides thereof must be electrically connected by soldering or the like. Therefore, the number of the connection points is increased, and thus, a problem occurs with respect to insufficient reliability. Also, since the ferromagnetic body 1 and the dielectric body 5 are fired separately, a plurality of firing steps must be carried out, and a troublesome assembly process is required, thereby making it difficult to reduce the manufacturing cost.
Therefore, the above problems can be possibly solved by simultaneously firing the ferromagnetic body 1 and the dielectric body 5. Namely, the above problems can be possibly solved by a method in which a green sheet for forming the ferromagnetic body 1 and a green sheet for forming the dielectric body 5 are laminated and simultaneously fired.
However, firing conditions for the ferromagnetic body 1 and the dielectric body 5 are different, and thus firing under conditions suitable for one of the bodies causes the possibility that firing of the other does not sufficiently proceed. Also firing under conditions intermediate between the conditions for both bodies causes a problem in that both the ferromagnetic body 1 and the dielectric body 5 may not be properly fired.
In addition, even if the ferromagnetic body 1 and the dielectric body 5 can be simultaneously fired, it is still not possible to use the same line in the step of preparing raw materials, thereby causing difficulties in decreasing the manufacturing cost.
Therefore, as a method of solving the above problems, a method has been proposed in which a central conductor arrangement portion and a capacitor formation portion for forming a matching circuit are formed in the same ferromagnetic body. This method will be described with reference to FIGS. 4 and 5.
FIG. 4 is an exploded perspective view illustrating another example of a conventional nonreciprocal circuit device. In the ferromagnetic body 15 shown in FIG. 4, a plurality of central conductors and a matching circuit are arranged. The electrode structure in the ferromagnetic body 15 is shown in an exploded perspective view of FIG. 5.
In the ferromagnetic body 15, ferromagnetic layers 15a to 15e are laminated. On the upper sides of the ferromagnetic layers 15b to 15d are formed a plurality of central conductors 16a, 16b to 16e and 16f, like the case of the ferromagnetic body 1 shown in FIG. 2. In this structure, ends of the central conductors 16a and 16b on the upper surface of the ferromagnetic layer 15b are electrically connected to a capacity electrode 17a. Similarly, ends of the central conductors 16c and 16d on the upper side of the ferromagnetic layer 15c are connected to a capacity electrode 17b formed thereon, and ends of the central conductors 16e and 16f on upper side of the ferromagnetic layer 15d are electrically connected to a capacity electrode 17c formed thereon.
On the upper sides of the ferromagnetic layers 15a and 15e are formed earth electrodes 18a and 18b, respectively. Therefore, in the ferromagnetic material 15 formed by laminating the ferromagnetic layers 15a to 15e and integrally firing the layers, not only the plurality of central conductors 16a to 16f are arranged, but also the capacity electrodes 17a to 17c for forming a matching circuit are arranged. The capacity electrodes 17a to 17c are overlapped with the earth electrodes 18a and 18b to form capacitors.
Referring to FIG. 4, the ferromagnetic body 15 is inserted into a concave 9a of a terminal plate 9 with a permanent magnet 11 disposed thereon, and held between metallic yokes 12 and 13 to form a nonreciprocal circuit device.
The nonreciprocal circuit device shown in FIG. 4 comprises the portion where the plurality of central conductors are arranged by using the ferromagnetic body 15, and the matching circuit. Therefore, the assembly step can be simplified, and the manufacturing cost can be decreased because a plurality of lines need not be used in the raw material preparing step. Also, since there is no need for joining the central conductors and the matching circuit by soldering or the like, the reliability can be improved.
However, since the capacitors for forming the matching circuit are formed by the ferromagnetic body 15, it is possible that a loss in the matching circuit may be increased due to the magnetic loss of the ferromagnetic body, whereby the insertion loss of the nonreciprocal circuit device may be increased.